To quickly and efficiently subject foamed polystyrene, etc., to a volume-reducing treatment to easily recover the highly pure polystyrene, etc., by immersing the material to be treated in a specific volume-reducing agent heated at the softening temperature of the material or higher.

This method for subjecting foamed polystyrene, etc., to a volume-reducing treatment comprises immersing the material to be treated, such as the foamed polystyrene, in a volume-reducing agent which comprises an aliphatic polyhydric alcohol and which is heated at the softening temperature of the material of higher, preferably at a higher temperature by about 40-70°C than the softening temperature. The aliphatic polyhydric alcohol is preferably e.g. glycerol, a polyethylene glycol including diethylene glycol and triethylene glycol, or trimethylol propane. The aliphatic polyhydric alcohol is preferably mixed with the carbonate, borate, sulfate or phosphate of an alkali metal, the chloride or borate of an alkali (ne earth) metal, triethanolamine, etc. E.g. when the foamed polystyrene is immersed in the heated volume-reducing agent, the foamed polystyrene is reduced into a volume of about 1/100.